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When you make an engine that runs at very high temperature, such as a jet engine, it is critical to select the right materials from which to make the engine parts. In 1903, the Wright brothers built an alu- minum block engine because of its light weight com- pared to cast iron. Its melting point of 660 °C was well above the engine’s operating temperature, so for them it was a good choice. However, aluminum could not be used in the hotter parts of a turbine engine where tem- peratures reach of 1800 °C or more because it would melt. Wright brother’s aluminum block engine. For turbines, engineers must not only choose high-temperature materials, but materials able to with- stand high-temperature oxidation and strong enough to maintain their shape while spinning at many revo- lutions per second. The specific requirements for the materials used in the engine are dictated by the condi- tions experienced in each part of the engine (see the Turbine Engines section for details). At the front of turbofan engines is a large fan. This typically does not get very hot (<150 °C) so aluminum, titanium, or stainless steel are all suitable for the fan blades. Most engines use titanium because it has a high strength-to-weight ratio, is corrosion and fatigue resistant, and would be able to withstand the impact of a bird strike. Next in line is the compressor section. Here the spinning blades of the compressor push the incoming air against the nonmoving stator vanes, which raises both the pressure and temperature of the air. The pres- sure of the air can be raised up to 30 times and the temperature, depending on the number of stages in the compressor, can rise to 1000 °C. Here the materi- als must have high strength at high temperatures; must resist fatigue, cracking, and oxidation; and also must resist “creep.” Creep is the tendency of a material to slowly change shape when stressed at high tempera- ture. Since no single metal would have all the desired properties, an alloy (a mixture of metals) is used. Very- high-temperature alloys are called superalloys and are generally nickel-, cobalt-, or iron-based alloys. Alumi- num and/or titanium are added for strength, and chro- mium, as well as rare earth elements like yttrium, are added to improve corrosion resistance. After the air is compressed, it enters the combus- tion chamber where fuel is added and burned. Here the temperatures can exceed 1800 °C and again superalloys are used, but without the titanium or alu- minum for strength because there are no moving parts. Instead, refractory metals are often added to a super- alloy. These are metals of unusually high resistance to heat, corrosion, and wear such as tungsten, molybde- num, niobium, tantalum, and rhenium. They are used in alloys and not as pure metals because they are among the densest of all the elements, a negative property when it comes to aircraft that need to keep weight to a minimum. Ceramics and ceramic-metal mixes are also used here because of their high heat resistance. We are familiar with pottery, tile, crucibles, and fire bricks as types of ceramics. They have very high melting points and don’t require the cooling systems like those needed to keep metals from melting so they make for lighter, less complicated engine parts. The down side is that they tend to fracture under stress, so engineers seek to create new ceramics composites that incorpo- rate other materials to improve properties. As the hot gases move toward the rear of the engine they cause the turbine to spin. This drives the fan and compressor by means of connected shafts to keep the engine operating. The first set of turbine blades are in the highest pressure, hottest part of the gas flow and are generally made of nickel-based- superalloy or ceramic blades. Unheated outside air is circulated through channels inside of the turbine blades to keep them from melting in this extreme environment. Further down the engine lower pressure turbine blades High-Temperature Materials Pushing the Envelope: A NASA Guide to Engines 41 CHEMISTRYPDF Image | NASA Guide to Engines
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